Abstract
This chapter presents recent mechatronics developments to create original terrestrial mobile robots capable of crossing obstacles and maintaining their stability on irregular grounds. Obstacle crossing is both considered at low and high speeds. The developed robots use wheeled propulsion, efficient on smooth grounds, and improve performance on irregular grounds with additional mobilities, bringing them closer to legged locomotion (hybrid locomotion). Two sections are dedicated to low speed obstacle crossing. Section two presents an original mobile robot combining four actuated wheels with an articulated frame to improve obstacle climbing. Section three extends this work to a new concept of modular poly-robot for agile transport of long payloads. The last two sections deal with high-speed motion. Section four describes new suspensions with four mobilities that maintain pitch stability of vehicles crossing obstacles at high speed. After the shock, section five demonstrates stable pitch control during ballistic phase by accelerating-braking the wheels in flight.
Top1. Mechatronic Design Of Mobile Robots For Obstacle Crossing
We are currently seeing a strong expansion of flying drones (UAVs, Unmanned Aerial Vehicles) of every sizes for professional activities and leisure. Although some of them are strong enough to carry a small payload, most of them are inexpensive light robots only equipped with vision sensors for tasks related to aerial inspection.
However, the majority of human activities are located on the ground and terrestrial mobile robots have a higher potential for helping humans in a convincing way, with a longer autonomy. Many tasks are becoming possible, such as transport on unstructured grounds or fast inspection by fleets of small agile robots. Civil and military service applications can be imagined for agriculture, forestry, transport, disabled people, industry, defence and crisis management during natural catastrophes.
One difficulty that prevents the extension of terrestrial mobile robots, compared to flying robots, is the varied nature of ground environments. For example, they can be structured or non-structured, flat or irregular, with cohesive or granular materials. Mobile robots are already well known in industry, where they move easily on structured flat cohesive grounds, guided by referenced landmarks. For example, Automatic Guided Vehicles (AGVs) are commonly used for transporting large parts for aeronautics and performing logistics tasks. However, as soon as the environment is natural, with irregular surfaces and granular grounds, without regular roadways and reference points, terrestrial mobile robots have difficulties to move and to perform their task.
Key Terms in this Chapter
Unstructured Environment: Environment that contains many obstacles and where vehicle localization is difficult. Most natural environments are unstructured.
Axle: A bar or shaft on which a wheel, a pair of wheels, or other rotating members revolve.
Wheel: A circular component, in contact with the ground at its periphery, allowing vehicle locomotion when it rotates around its axis of revolution with respect to a vehicle frame. Wheels facilitate the motion of vehicles and can support a vertical load during transport of heavy objects. They can be rigid of soft, in one part or several. Their shape is variable and can look like a cylinder or torus. They can include a tyre at their periphery.
Structural Synthesis: The design process that aims at finding mechanical kinematics suitable to given structural parameters (mobility, connectivity, redundancy and overconstraint degrees). This process occurs before dimensional synthesis.
Stability Margin: A numeric indicator to measure the risk of tip-over of a vehicle or mobile robot. It can be calculated for statics or dynamics. Dozens of stability margins exist, according to considered vehicle and locomotion mode ( Gonzalez de Santos et al. , 2006 ; Chebab, 2013 ).
Kinematic Chain: System of interconnected links permitting relative motion of any link with respect to the remaining links.
Skid-Steering: Steering manoeuvre where vehicle rotation is obtained by a difference in speeds of left and right wheels, but at the price of lateral slippage. It is typical for vehicles with non-orientable wheels.
Roll Angle: Rotation angle of the vehicle frame around a longitudinal axis, in the locomotion direction, generally denoted by X axis.
Tyre: A rubber covering, typically inflated or surrounding an inflated inner tube, placed round a wheel to form a soft contact with the road.
Differential Steering: See Skid-steering.
All-Terrain: Qualification of a vehicle or a locomotion mode suitable to evolve in every type of environments, including unstructured environments.
Hybrid Locomotion: A general term to describe a type of locomotion using combinations of classical locomotion modes and propulsion devices such as wheels, tracks, or legs.
AGV: Automated Guided Vehicle. Another name for a mobile robot, often used in logistics.
Cooperative Robot: Robot able to cooperate with other robots of similar or different architectures, or even with human operators, to jointly perform a common task. An example of task could be co-manipulation.
Locomotion Mode: The set of methods, means and media used by a biological or artificial system to generate its displacement in its environment.
Leg: Linkage connecting the body of a walking mobile robot to the ground. The last link in contact with the ground is named “foot”.
Dimensional Synthesis: The design process that, after structural synthesis, enriches kinematics with additional attributes (lengths, materials...) to optimize kinematic properties (trajectories, orientations, workspaces...) and structural properties (strength, stiffness...)
Parallel Mechanism: Mechanism with a driven link connected to the frame by means of at least two kinematic chains.
Pitch Stabilisation: A control method to maintain a vehicle pitch angle in an angular range suitable for proper locomotion on ground or safe landing in case of aerial phases.
Pitch Angle: Rotation angle of the vehicle frame around a lateral axis, orthogonal to the longitudinal and vertical directions, generally denoted by Y axis.
Vehicle: A mobile machine that transports passengers or payload.
Propulsion Device: Mechanism that is used for vehicle locomotion and includes one or several links in contact with the ground. Classical propulsion devices include wheels, tracks and legs.
Agile Robot: Robot able to quickly move or change its configuration in order to adapt itself to the environment or to the task requirements.
Robot: Mechanical system under automatic control that performs operations such as handling and locomotion.
Ackerman Steering: Steering manoeuvre obtained with a wheeled vehicle where all the wheels have the same centre of steering-rotation. It can be obtained if all the wheel-axes cross at the same point during steering, which guarantees a null lateral slippage. This condition is also known in France as “épure de Jeantaud”.
Obstacle: Object or feature of the environment that blocks locomotion.
Payload: Actual weight of the cargo carried by a transport vehicle. Also used in this work to name the manipulated cargo during co-manipulation.
Structured Environment: Environment that has been modified to remove a sufficient number of obstacles and provide enough localization means to facilitate locomotion and tasks in general.
Yaw Angle: Rotation angle of the vehicle frame around the vertical axis, generally denoted by Z axis.
Suspension Mechanism: A mechanism, with one degree of freedom or more, which connects the vehicle frame to one or several wheels (or other propulsion devices). It aims to control the vehicle frame motion during locomotion on irregular grounds by using passive or active devices.
ZMP: Zero Moment Point. A point used for dynamics of legged locomotion, located at the foot-ground contact, and where reaction forces from the ground do not generate any moment around a horizontal axis.
Rocker-Bogie: Suspension arrangement used in the Mars rovers and patented by NASA ( Bickler, 1988 ). Bogies are sub-assemblies of wheels that can have a pitch angle with respect to the frame. This configuration is useful to ensure that all the wheels of the robot (originally six) remain in contact with irregular grounds in unstructured environments.
Mobile Robot: Robot with a locomotion capacity. It is an automated vehicle. Its functions include exploration (locomotion only), transport of payloads or more complex tasks to perform with onboard systems, such as robotic arms.
Serial Mechanism: Mechanism with a driven link connected to the frame by means of a serial kinematic chain (series of consecutive links, each being connected to the next neighbour by a single joint).
UAV: Unmanned Aerial Vehicle, commonly known as a flying drone. Not to be confused with AUV (Autonomous Underwater Vehicle).
Co-Manipulation: Task during which several entities (that can be human or robotic) jointly manipulate the same payload. A robot developed to collaborate with humans is named a “cobot”.
Track: Closed band around the wheels of certain vehicles such as tanks, intended to facilitate movement over rough or soft ground. A track can be made of one single deformable band or a chain of articulated links.